A sensor of the kind described above is disclosed in U.S. Pat. No. 4,609,966 incorporated herein by reference. In this sensor, a stationary capacitor electrode is applied to a substrate and a movable membrane electrode is attached opposite to the stationary capacitor electrode at a predetermined spacing therefrom. The two electrodes enclose a chamber with their respective edges which is sealed with respect to the gas atmosphere to be measured. The chamber is under a non-changeable pressure. The movable membrane is subjected to the gas atmosphere to be measured so that under changing pressure conditions it takes on a spacing in correspondence thereto with respect to the stationary electrode. The movements of the elastic membrane are taken up as capacitance changes and are processed as a signal for the pressure of the gas to be investigated.
In this known configuration, the quality of the measurement is dependent upon the movability of the membrane which, in turn, is a function of the membrane thickness. In order to be able to measure even slight pressure changes, a membrane as thin as possible is required which however is sensitive with respect to mechanical damage. A deformable membrane has the disadvantage with respect to making measurements that it displays flow characteristics under the force effect of the gas to be investigated so that its measuring characteristic changes over a long period of time in a manner which cannot be predicted. This becomes manifest in an undesired drift behavior of the measurement signal.
Furthermore, the requirement must be satisfied that the sealed chamber does not develop any leaks during the long-term use of the sensor. Considerable difficulties are encountered in fixing a spacing for the quiescent condition which remains constant over the length of the elastic membrane. Membrane spacings in the micrometer range can be achieved only with difficulty and above all cannot be maintained constant. Because of their capacitive effect, these membrane distances could otherwise lead to small sizes of the apparatus.